Since its inception in 2013, the CERN Neutrino Platform has evolved into a worldwide hub for both experimental and theoretical neutrino physics. Besides its multifaceted activities in hardware development – including most notably the ProtoDUNE detectors for the international long-baseline neutrino programme in the US – the platform also hosts a vibrant group of theorists.
From 13 to 17 March this group once again hosted the CERN Neutrino Platform Pheno Week, after a COVID-related hiatus of more than three years. With about 100 in-person participants and 200 more on Zoom, the meeting has become one of the largest in the field – a testament to the ever-growing popularity of neutrinos among particle physicists, even though neutrinos are the most elusive among all known elementary particles.
Talks at the March event reflected the full breadth of the subject, with the first days devoted to novel theoretical models explaining the peculiar relations observed among neutrino masses and mixing angles, and to understanding the way in which neutrinos interact with nuclei. The latter topic is particularly complex, given the vast range of energies in which neutrinos are studied – from non-relativistic cosmic background neutrinos with sub-meV energies to PeV-scale neutrinos observed in neutrino telescopes. An especially popular topic has also been the possibility of discovering physics beyond the Standard Model in the neutrino sector. In fact, because of their ability to mix with hypothetical “dark sector” fermions – that is, fermions potentially related to the physics of dark matter, or even dark matter itself – neutrinos offer a unique window to new physics.
The second part of the workshop was devoted to the neutrino’s role in astrophysics and cosmology. “There’s actually a two-way relationship between neutrinos and the cosmos,” explained invited speaker John Beacom (Ohio State University). “On the one hand, astrophysical and cosmological observations can teach us a lot about neutrino properties. On the other, neutrinos are unique cosmic messengers, and from observations at neutrino telescopes we can learn fascinating things about stars, galaxies and the evolution of the universe.” In recent years, for instance, neutrinos have allowed physicists to shed new light on the century-old problem of where ultra-high-energy cosmic rays come from. And the next galactic supernova – an event that happens on average every 30 to 100 years – will be a treasure trove of new information, given that we expect to observe tens of thousands of neutrinos from such an event. At the same time, cosmology sets the strongest upper limits on the absolute scale of neutrino masses, and with the next generation of cosmological surveys we have every expectation to achieve an actual measurement of this quantity. This is interesting because neutrino oscillations, while establishing that neutrinos have non-zero mass, are only sensitive to differences of squared masses, not to the absolute mass scale.
The programme of the Neutrino Platform Pheno Week closed with a tour of the ProtoDUNE experiments, giving the mostly theory-oriented audience an impression of how the magnificent machines testing our theories of the neutrino sector are being developed and assembled.